Chromium-containing bodies of improved resistance to oxidation and nitrification

ABSTRACT

CHROMIUM-CONTAINING ALLOY SINTERED BODIES HAVING IMPROVED RESISTANCE TO OXIDATION AND NITRIFICATION ARE PRODUCED BY A PROCESS WHICH INCLUDES THE STEP OF HEATING THE BODIES TO AN ELEVATED TEMPERATURE IN THE PRESENCE OF FREE OXYGEN AND THEREBY FORMING A CR2O3 SCALE ON THE BODIES AND A CHROMINUM ION MIGRATION-BLOCKING LAYER OF YCRO3 AT THE INTERFACE OF THE CR2O3 SCALE AND THE METAL BODIES.

Nov. 21, 1972 U SEYBOL A. CHROMIUM-CONTAINING BODIES 0F IMPROVED RESISTANCE T0 OXIDATION AND NITRIFICATION Original Filed Jan. 17, 1967 g I- E I. o x h (L I I I I I I I I I I I I I I I I 9 E- a can/6w Inventor His A from ey- Alan u ISjI bolr United States Patent 3,703,369 CHROMIUM-CONTAINING BODIES OF IMPROVED III IFggTANCE TO OXIDATION AND NITRIFICA- Alan U. Seybolt, Ballston Spa, N.Y., assignor to General Electric Company Application Jan. 17, 1967, Ser. No. 609,931, which is a continuation-in-part of application Ser. No. 524,439, Feb. 2, 1966. Divided and this application Dec. 5, 1969, Ser. No. 882,726

Int. Cl. B221 1/00 U.S. CI. 75206 3 Claims ABSTRACT OF THE DISCLOSURE Chromium-containing alloy sintered bodies having improved resistance to oxidation and nitrification are pro duced by a process which includes the step of heating the bodies to an elevated temperature in the presence of free oxygen and thereby forming a Cr O scale on the bodies and a chromium ion migration-blocking layer of YCrO at the interface of the Cr O scale and the metal bodies.

This application is a division of my copending application Ser. No. 609,931 filed Jan. 17, 1967 now abandoned, as a continuation-in-part of my application Ser. No. 524; 439, filed Feb. 2, 1966 (now abandoned) and assigned to the same assignee as the present invention.

It is known and recognized that many metals depend upon the presence of a tightly adherent continuous oxide film to achieve resistance against attack either by oxygen or other gaseous or liquid materials. One such metal is chromium which depends upon the formation of a Cr O film on the surface in order to be protected against further oxidation and nitrification, particularly at elevated temperatures. Due to the great afiinity of this element for oxygen, the initial oxidation rate is fairly rapid, but once a protective film has been formed further oxidation occurs at increasingly slow rates because the rate of growth of Cr O is controlled by the diffusion rate of Cr ions through the thickening film. Nitrification can occur only by passage of nitrogen through cracks or discontinuities in the Cr O scale. Obviously, there is an increase in both oxidation and nitrification of chromium as the temperature of the metal is raised. This same sort of protective mechanism is involved where chromium is alloyed in significant percentages with other material such as iron, nickel and cobalt.

It is a principal object of this invention to provide chromium or chromium-containing sintered alloy bodies having improved resistance to oxidation and nitrification. It is an additional object of this invention to provide a process for producing bodies containing significant percentages of chromium which have improved resistance to oxidation and nitrification.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

In the drawings:

The figure is a graph of oxidation rate versus time showing the improved resistance to oxidation of the bodies of this invention.

Broadly, the present invention involves bodies of chromium base or of iron, nickel or cobalt base, or combinations of these metals with each other which contain significant percentages of chromium. These bodies are produced by powder metallurgy techniques and have improved resistance to high temperature oxidation and nitrification. The bodies are made of either pure chromium or of iron, nickel, cobalt or combinations of these combined with each other which contain from 15 to 30 weight percent chromium. To whichever base metal is selected, is added one to five volume percent of yttrium oxide (Y O It has been found that the yttrium oxide, which is present initially as a finely-divided, evenly dispersed particulate phase, is effective in reducing high temperature oxidation and nitrification of the principal constituent or matrix metal from which the bodies are constructed.

The process of this invention includes the novel step of forming a Cr O scale on the sintered body and simultaneously forming a layer of YCrO at the interface of the Cr O scale and the metal body surface. This step is carried out by heating the body in air at an elevated temperature such as 1150 C.

Chromium oxide, in order to be an effective protective layer, must be free of breaks and also remain tightly adherent to the parent body. Any cracks in or spalling away of the oxide layer would obviously expose fresh metal for further attack by the environment in which the body is situated. Since metals of the type with which this appli cation is concerned find use in gas turbine operations, it is apparent that the formation of a good protective film is virtually necessary to the attainment of a reasonable operating life. The problem encountered with regard to continued oxidation is obvious but the problems encountered with respect to nitrification are easily as important.

It is well known that chromium alloys containing as little as about 20 weight percent chromium are susceptible to chromium nitride formation during heating in air. At elevated temperatures, nitrogen penetrates the chromium oxide layer present on the surface and forms chromium nitride (Cr N). This formation of Cr N is very detrimental to the structural integrity of high chromium alloys (and chromium), since sheet materials of ordinary thicknesses can be completely penetrated within a few hours at high temperatures.

I have now found that the addition of one to five volume percent yttrium oxide (Y O to the chromium bearing base metals can efficiently and effectively reduce both the oxidation and nitrification rates. This result is attributable to a unique and hitherto unknown and unrecognized property of yttria. Specifically, I have discovered that yttria in a chromium base alloy will diffuse to the surface of the alloy body and penetrate into Cr O scale on the body and additionally will react with the Cr O scale to form a layer of YCrO at the interface of the body and the scale. This YCrO layer is impenetrable by nitrogen and also inhibits migration of chromium ions from the body into the Cr O scale. The presence of yttrium ions in the Cr O scale has been observed through X-ray fluorescence. Observation of the YCrO structure may be made by X-ray diffraction and the location of the YCrO layer underlying the Cr O scale can be observed through metallographic technique.

Again, the base metals of this invention are iron, nickel, cobalt and combinations of these with each other which contain from 15 to 30 weight percent chromium, 20 to 25 percent being preferred. Compositions of particular significance are iron containing 15 to 30 weight percent chromium and iron combined with from 15 to 25 weight percent chromium and from 5 to 20 weight percent nickel. These metal alloys all depend upon the presence of the oxide film and are therefore benefitted by the presence of yttrium oxide in the sintered metal compact. Specific alloys having significantly improved oxidation resistance are 1) 20 percent Cr, 5 volume percent Y O balance nickel;

(2) 20 percent Cr, 5 volume percent Y O balance Co;

(3) 20 percent Cr, 1 volume percent Y O balance Co;

(4) 20 percent Cr, 2.5 volume percent Y O balance Ni.

In these compositions, combinations of iron and nickel can be substituted for either nickel or cobalt alone.

While the materials of this invention have thus far been described only in terms of iron, nickel or cobalt bases containing from 15-30 percent Cr and from 1-5 volume percent Y it should be pointed out that additional elements could be added to achieve particular physical properties. For example, elements such as molybdenum or tungsten can be added to obtain bodies capable of solid solution hardening. Or, elements like titanium or aluminum can be added to achieve precipitation hardening in the alloys. Generally, these special agents are added in amounts that will not deleteriously affect the nature of the chromium oxide film that is formed on the surface of the bodies. Twenty to twenty five percent of the special agent would normally be considered as a maximum addition to the base metal.

Bodies according to this invention can be produced by preparing the selected quantity of base metal powder, the powder being no larger than 200 mesh and preferably of 325 mesh or smaller, and then adding to it from one to five volume percent of Y O which may be of particle size from two to 75 microns. The powdered ingredients are thoroughly milled together until complete dissemination of the yttrium oxide throughout the particulate base metal is accomplished. Thereafter, the powder is placed within a suitable compacting apparatus and pressed and fired to arrive at a final body of essentially complete density, that is free from porosity, in which yttrium oxide is present as a finely-divided, evenly dispersed particulate phase.

In accordance with this invention, it is important that the yttria be present in amount and form such that the YCrO barrier layer can be formed by reaction of yttria with the Cr O scale. Thus at least one volume percent yttria is essential in the present new compositions and the yttria must be distributed substantially uniformly through the metal oxide mixture so that the presence of yttria throughout the surface portion of a formed body of the mixture is assured. The particle size of the yttria is important in that a gross difierence in size between the metal and the oxide of the mixture complicates the task of distributing the oxide uniformly through the metal powder. As a practical matter, the metal powder used will be of particle size from 10 to 75 microns and the yttria will be about the same, although yttria of particle size as small as 2 microns may be used without creating special mixing problems. The use of more than five volume percent yttria, regardless of particle size, affords no advantage and could have a detrimental effect upon the desired properties of the ultimate sintered powder metal body.

As an example of the present invention, a chromium body was prepared by mixing 99.9 percent pure chromium powder of 325 mesh (i.e. 40-rnicron size) with approximately 5 volume percent yttria powder of about 2 micron size in a ball mill. The ball milling was effected for about 250 hours in a tungsten carbide mill using tungsten carbide balls and toluene as a liquid. The chromium powder, while undergoing some reduction in size, was principally flattened to about 5 microns thickness during the milling operation. The powder mixture was airdried and pressed into a two inch 0.1). mild steel extrusion can and extruded through a one-half inch diameter die at 1100 C. to provide a 16 to 1 reduction. The steel can was removed and a. chromium-yttrium oxide alloy rod free of porosity was obtained. Rectangular oxidation samples were then prepared which had about four square centimeters of surface area. Surface preparation of the samples consisting of polishing through 600 silicon carbide paper ended by final cleaning with toluene and acetone.

To ascertain the manner in which oxidation resistance had been effected by the addition of 0 samples weighing about one-half gram were suspended from a spring balance which, with a cathetometer, provided a means of continuous weighing, the sensitivity being about 0.3 milligram. A furnace equipped with a 96 percent alumina tube was first heated to the desired temperature and then the sample was inserted. A slow flow (approximately 200 cc. per minute) of dry air was started and readings taken indicated the weight gain of the material.

Referring to the figure of the drawings, the exposure time is plotted on a log-log scale in minutes against the weight gain in milligrams per square centimeter. It will be noted that all samples closely followed the parabolic rate constant characteristic of pure chromium for about the first 200 to 300 minutes. The dotted curve 10 indicates the parabolic oxidation rate constant of pure chromium. After about the first 200 to 300 minutes, however, the rate constant decreased for samples which contained yttria. The curve 11 indicates samples tested at 1250" C. while the curve 12 indicates the rate constant for samples tested at 1150" C. It is apparent from these curves that the addition of small percentages of yttrium oxide to the chromium has significantly improved the oxidation resistance at elevated temperatures.

The specimens were also studied metallographically to determine What the effect of the yttrium oxide was in suppressing the formation of Cr N. From these studies, it was found that chromium containing five volume percent yttria formed no brittle nitride even after an air exposure of many times that of pure chromium. This circumstance is attributable to the nature of the modified Cr O- scale which dissolves some Y O and also forms the double oxide YCrO at the C50 alloy interface. The YCrO forms a blocking layer preventing migration of chromium ions through the Cr O scale. This modified Cr O scale offers a more complete barrier to nitrogen, which evidently cannot diffuse through the scale but must gain entrance through cracks or other discontinuities in the Cr O layer.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for producing sintered powder metal bodies comprising the steps of preparing an intimate particulate mixture consisting essentially of 15 to 30 weight percent chromium, l to 5 volume percent yttria, balance a metal selected from the group consisting of iron, nickel and cobalt, the particle size of the constituent metals being not larger than about 200 mesh, compacting and firing the mixture to form a body free from porosity, and forming a Cr O scale on the resulting body and a chromium ion migration-blocking layer of YCrO; at the interface of the Cr O scale and the metal body surface by subjecting said body to contact with free oxygen at an elevated temperature.

2. The process of claim 1 in which the particle size of the constituent metals and metal oxide is not greater than 325 mesh and the final step is carried out by heating the body in air to a temperature of 1150 C.

3. The process of claim 1 in which compacting and heating of the powder mixture are accomplished by pressing the mixture in a steel extrusion can and then extruding the resulting jacketed compact through a die at 1100 C. to achieve a 16 to 1 reduction, and then removing the steel can, and in which the final step is carried out by heating the resulting compact powder metal body in air at 1150 C.

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